This invention relates to the hydraulic fracturing of high temperature subterranean formations. More particularly it relates to a method for fracturing subterranean formations penetrated by a well bore, wherein a fluid composition is injected into a formation through a suitable conduit at a rate and pressure sufficient to produce a fracture in the formation. It is well known that production in petroleum, natural gas and geothermal wells can be greatly enhanced by hydraulic fracturing techniques. These techniques are known in the art and generally comprise introducing an aqueous solution of a water-soluble polymer (e.g. Guar Gum) in which "proppants" (e.g. coarse sand or sintered bauxite or synthetic ceramic materials) are suspended through the well bore under extremely high pressures into the rock structure in which the petroleum, gas or steam is entrained. Minute fissures in the rock are thereby created and held open by the suspended particles after the liquid has drained off. The petroleum, gas or steam can then flow through the porous zone into the well. Polysaccharides e.g. guar and guar derivatives are the most commonly used water-soluble polymers for hydraulic fracturing.
Aqueous solutions of guar and guar derivatives develop increased viscosity upon the addition of various metal ions. Viscoelastic gels are formed by the chemical linking or cross-linking of two or more polymer chains. The result is a more ordered network structure which increases the effective molecular weight and thereby, the viscosity. The stability of these high viscosity cross-linked gels is dependent on many factors including pH and temperature. The viscosity stability of water-soluble polymer solutions as a function of time and temperature, is crucial for successful applications in the oil field area. Thermal stability is a major factor in selecting a water-soluble polymer for wells having high bottom-hole temperatures. It is well known that crosslinked fracturing fluids degrade with time as a function of temperature and shear, resulting in a loss of viscosity and proppant carrying ability in a short time at temperatures of 250.degree. F. and above.
The observed loss of viscosity as a function of time, temperature and shear, is the result of degradation by several pathways, for example chemical, biological, mechanical and radiation. Biological degradation can be minimized by the proper choice of biocide. Mechanical degradation is the result of applying a critical stress to the gel, resulting in chain scission. Its effect can be minimized by the use of properly engineered surface equipment, etc. Radiation of a polymer solution results in the formation of hydroxyl radicals which abstract hydrogen from the polysaccharide. These radicals trigger oxidation or the formation of intermediates which are easily hydrolyzed. Exposure to radiation, however, cannot usually by controlled and is dependent on time.
There are two chemical pathways of importance. One is hydrolysis of the glycosidic linkage, resulting in scission of the polysaccharide chain. The other is oxidative/reductive depolymerization. Acid catalyzed hydrolysis of the glycosidic bond is well documented. The rate of degradation by glycosidic bond hydrolysis is dependant on reaction time, system pH and temperature. Oxidative/reductive depolymerazations involve the oxidation of the polysaccharide by a radical pathway in the presence of oxygen. Transition metal ions, (e.g. iron) can promote these processes. This thermal degradation of the gels can be minimized by the addition of oxygen scavengers such as sodium thiosulfite, methanol, thiourea and sodium thiosulfate and by avoiding extremely high or low pH conditions. Previously, hydraulic fracturing fluids were pumped at pH's of from 6 to 9.
In order to maintain proppant carrying viscosities in fracturing fluids at this pH in high temperature formations where thermal degradation occurs, polymer loadings were typically increased by 50 to 100 percent. It has been common practice to pump fluids containing 60 pounds of gelling agent per thousand gallons of fluid in order to maintain sufficient viscosity at high temperatures, e.g. 300.degree. F. With the process of the present invention, as little as 30 to 40 pounds of gelling agent are needed. Increased polymer loadings not only increase the cost of the fracturing fluid but also increase the amount of insoluble material or residue remaining in the formation after the fracturing operation is complete. Residue remaining in the formation damages the formation by decreasing the permeability or conductivity of the formation and proppant pack thus reducing oil or gas recovery. The amount of residue remaining in a formation can range from as high as 7.5 percent by weight for unmodified guar down to 2 to 3 percent for hydroxypropylguar and as low as 1 to 2 percent for carboxymethylhydroxypropylguar. Formation damage caused by gel residues is a significant factor affecting productivity of a well especially when greater quantities of gel are used to offset thermal degradation.